With the popularization of the Internet in recent years, a greater capacity in trunk-line optical communication systems is in demand. Optical transmitters-receivers that are capable of transmitting signals exceeding 40 Gbit/s or 100 Gbit/s per wavelength are being researched and developed.
However, increasing the transmission capacity (bit rate) per wavelength leads to a significant decrease in the optical-signal-verses-noise (OSNR) ratio, resulting in significant deterioration of signal quality due to waveform distortion caused by, for example, wavelength dispersion of transmission paths, polarization mode dispersion, or a nonlinear effect.
Therefore, for optical communication systems exceeding 40 Gbit/s, digital-coherent receiving methods that have great tolerance against the optical-signal-verses-noise (OSNR) ratio and the waveform distortion of transmission paths are attracting attention. For example, see Dany-Sebastien, Ly-Gagnon, et al., “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, Journal of Lightwave Technology, Vol. 24, No. 1, 2006, pp. 12-21.
In the receiving methods of the related art, the on/off states of the optical power are assigned to binary signals and are directly detected. In contrast, in a digital-coherent receiving method, the optical power and the phase information of light are extracted by a coherent receiving system, and the extracted optical power and phase information are quantized by an analog/digital converter (ADC) so as to be demodulated in a digital-signal processing circuit.
Since a digital-coherent receiving method allows for improved tolerance against the OSNR by the coherent receiving method as well as intensive waveform-distortion compensation by the digital-signal processing circuit, high performance can be achieved even in an optical transmitter-receiver with 40 Gbit/s or greater.
Furthermore, a digital-coherent receiving method can be combined with a modulation method in which multi-level information is transferrable in one symbol time. Known examples of such a modulation method include a multilevel modulation method, a polarization multiplexing method in which different pieces of information are multiplexed into orthogonal polarized waves, and a multicarrier multiplexing method in which different pieces of information are multiplexed into a plurality of frequencies (carriers) multiplexed at high density within a single wavelength grid. Typical examples of multicarrier multiplexing method include frequency division multiplexing (FDM) and orthogonal frequency division multiplexing (OFDM).
FIG. 24 illustrates a configuration example of a dual-polarization quadrature-phase-shift-keying (DP-QPSK) type digital-coherent optical transmitter-receiver (transceiver) that uses both the polarization multiplexing method in which different pieces of information are multiplexed into two orthogonal polarized waves (dual polarization) and a quadrature-phase-shift-keying (QPSK) method, which is a type of multilevel modulation method, in which quadrature information is transmitted in one symbol time.
An optical transmitter includes a transmission-signal generator 11, a signal light source (LD) 12, driver amplifiers 13 to 16, phase modulators 17 to 20, and a polarization beam combiner (PBC) 21, and transmits an optical signal to an optical receiver via a transmission path 22. The LD 12, the driver amplifiers 13 to 16, the phase modulators 17 to 20, and the PBC 21 constitute an electrical/optical conversion circuit 37.
The transmission-signal generator 11 outputs a transmission signal to the driver amplifiers 13 to 16. The driver amplifiers 13 to 16 amplify the transmission signal and output the amplified transmission signal to the phase modulators 17 to 20, respectively. The LD 12 outputs signal light to the phase modulators 17 to 20. The phase modulators 17 to 20 modulate the signal light with the transmission signal and output the signal light to the PBC 21. The light output from the phase modulators 17 and 18 and the light output from the phase modulators 19 and 20 have polarized waves that are orthogonal to each other. The PBC 21 combines the optical signals output from the phase modulators 17 to 20 and outputs the combined optical signal to the transmission path 22.
On the other hand, the optical receiver includes polarization beam splitters (PBSs) 23 and 24, a local light source (LD) 25, optical hybrids 26 and 27, photo-detectors (PDs) 28 to 31, ADCs 32 to 35, and a digital-signal processing circuit 36, and receives the optical signal from the transmission path 22. The PBSs 23 and 24, the LD 25, the optical hybrids 26 and 27, and the PDs 28 to 31 constitute a digital-coherent optical/electrical conversion circuit 38.
The PBS 23 splits the optical signal received from the transmission path 22 into two orthogonal polarized-wave components and outputs the two polarized-wave components to the optical hybrids 26 and 27, respectively. The PBS 24 splits local light output from the LD 25 into two orthogonal polarized-wave components and outputs the two polarized-wave components to the optical hybrids 26 and 27, respectively.
The optical hybrid 26 mixes the optical signal and the local light and outputs two orthogonal phase components to the PDs 28 and 29, respectively. Similarly, the optical hybrid 27 mixes the optical signal and the local light and outputs two orthogonal phase components to the PDs 30 and 31, respectively.
The PDs 28 and 29 perform photoelectric conversion to convert the optical signal into an electric signal and output the electric signal to the ADCs 32 and 33. Similarly, the PDs 30 and 31 convert the optical signal into an electric signal and output the electric signal to the ADCs 34 and 35. The digital signal output from the ADCs 32 and 33 includes the intensity information and the phase information of the optical signal input to the optical hybrid 26, and the digital signal output from the ADCs 34 and 35 includes the intensity information and the phase information of the optical signal input to the optical hybrid 27.
The digital-signal processing circuit 36 uses the digital signals output from the ADCs 32 to 35 to perform demodulation and waveform-distortion compensation on the received signals. This method of receiving polarized waves in two states is called polarization diversity reception. With the DP-QPSK method, transmission and reception of an optical signal can be performed by utilizing polarization diversity reception.
FIG. 25 illustrates a configuration example of a digital-coherent optical receiver that does not utilize polarization diversity reception. This optical receiver includes a polarization controller 41, an optical hybrid 42, an LD 43, PDs 44 and 45, ADCs 46 and 47, and a digital-signal processing circuit 48. The polarization controller 41, the optical hybrid 42, the LD 43, and the PDs 44 and 45 constitute a digital-coherent optical/electrical conversion circuit 49.
The polarization controller 41 changes the polarization state of a received optical signal so that it accords with the polarization state of local light output from the LD 43. The optical hybrid 42, the LD 43, the PDs 44 and 45, and the ADCs 46 and 47 operate in the same manner as the optical hybrid 26, the PDs 28 and 29, and the ADCs 32 and 33 illustrated in FIG. 24. The digital-signal processing circuit 48 uses digital signals output from the ADCs 46 and 47 to perform demodulation and waveform-distortion compensation on the received signals. In the case of a configuration that does not use polarization multiplexing, the reception may be performed using the configuration illustrated in FIG. 25.
A configuration illustrated in FIG. 26 may be used as an alternative to the configuration in FIG. 25. FIG. 26 illustrates a configuration example of a self-coherent optical receiver that does not use a local light source. This optical receiver includes an optical coupler 51, delay interferometers 52 and 53, PDs 54 to 56, ADCs 57 to 59, and a digital-signal processing circuit 60. The optical coupler 51, the delay interferometers 52 and 53, and the PDs 54 to 56 constitute a digital-coherent optical/electrical conversion circuit 61.
The optical coupler 51 divides a received optical signal into three components and outputs the three components to the delay interferometers 52 and 53 and the PD 56, respectively. The delay interferometers 52 and 53 extract phase components from the received optical signals by causing the optical signals to interfere with optical signals received earlier by one symbol time or a given time, and output the phase components to the PDs 54 and 55. The two phase components respectively output from the delay interferometers 52 and 53 are orthogonal to each other.
The PDs 54 to 56 convert the optical signal into an electric signal and output the electric signal to the ADCs 57 to 59. The digital-signal processing circuit 60 uses the digital signals output from the ADCs 57 to 59 to reconstitute the received optical signal, and performs demodulation and waveform-distortion compensation on the signal.
In addition to the QPSK method, other modulation methods can also be used in the digital-coherent optical receiver. Other examples of modulation methods include a non-return-to-zero (NRZ) modulation method, a return-to-zero (RZ) modulation method, an M-ary phase shift keying (M-PSK) modulation method, an M-ary quadrature amplitude modulation (M-QAM) method, an OFDM method, an FDM method, and a modulation method with a combination of these methods and polarization multiplexing.
As mentioned above, when the same transmission path is used, an increase in the bit rate per wavelength leads to deterioration in transmission performance. In particular, since an optical signal of 40 Gbit/s or greater has a large number of factors that can deteriorate the transmission performance due to an increase in bit rate, a trade-off between a greater transmission capacity per wavelength and the transmission distance becomes noticeable.
If an optical communication network can be made by flexibly selecting an optimal bit rate in accordance with the required transmission distance of the path and the condition of the path, an increase in a transmission capacity that is operable in the entire network can be expected.
However, in the related art, since the transmission characteristics significantly vary depending on each bit rate, an optical transmitter-receiver that can handle various bit rates is extremely expensive.
For example, a 10-Gbit/s optical transmitter-receiver employs an NRZ method. In contrast, in a 40-Gbit/s optical transmitter-receiver, the optical receiver requires an optical device used for compensating for waveform degradation, such as a wavelength-dispersion compensator or a PMD compensator, since the effect of waveform degradation caused by wavelength dispersion and polarization mode dispersion (PMD) is significant. In addition, in order to improve the OSNR tolerance, a phase modulation method such as a differential phase shift keying (DPSK) or a differential quadrature phase shift keying (DQPSK) is used as the modulation method.
In this case, it is assumed that an optical transmitter-receiver that can handle the two bit rates, 10 Gbit/s and 40 Gbit/s, is expensive.
On the other hand, a digital-coherent optical receiver is capable of compensating for linear distortion, such as wavelength dispersion of the transmission path or PMD, by using a digital-signal processing circuit, and the components other than the digital-signal processing circuit can have the same optical-receiver configuration regardless of the modulation method. Therefore, it is expected that the configuration of the optical receiver does not significantly change in accordance with the bit rate, whereby an optical transmitter-receiver that can handle various bit rates can be readily achieved.
However, in order for the digital-coherent optical receiver to have the capability to handle various bit rates, the following conditions need to be satisfied. Firstly, in order to handle various bit rates, a sampling clock source with a wide operational frequency range is required. Secondly, a digital-signal processing circuit that can operate even when the processing speed significantly varies is required. Thirdly, when the processing speed changes significantly, high-speed trackability with respect to a change in the quality of a received optical signal caused by polarization fluctuation or PMD fluctuation is required.